The present invention relates to a medical X-ray CT (Computed Tomography) apparatus, and to an X-ray CT apparatus which realizes a reduction in exposure and an improvement in image quality using an X-ray automatic exposure function upon a conventional scan (axial scan), a cine scan, a helical scan, a parameter-pitch helical scan or a helical shuttle scan.
An X-ray CT apparatus having a multi-row X-ray detector or a two-dimensional X-ray area detector of a matrix structure typified by a flat panel has heretofore realized a reduction in exposure and an improvement in image quality using an X-ray automatic exposure function (corresponding to a function called “auto milliampere” or the like) (refer to, for example, Japanese Unexamined Patent Publication No. 2001-178713).
Here, the term “X-ray automatic exposure function” is called as a function for automatically setting an X-ray tube current condition for the irradiation of X-rays to a subject corresponding to a irradiation position so as to uniform image quality characteristic value typified by a standard deviation of a CT value at a tomographic image continuous in a z-direction.
For example, the X-ray CT apparatus optimizes a set value of an X-ray tube current supplied to an X-ray tube while scanning so as to meet a sectional area (profile area) at each z-direction position, of the subject, and optimizes a set value of an X-ray tube current at the time that a data acquisition system is rotated once upon photography, so as to meet a flat degree or aspect ratio of the shape of the subject in an xy plane, thereby realizing a reduction in exposure and an improvement in image quality.
Described specifically, a scout scan is executed on a subject before execution of an actual scan for the subject to thereby image or photograph a scout image corresponding to a fluoroscopical image of the subject. Thereafter, a central processing unit calculates and sets respective tube current values supplied to the X-ray tube, based on the photographed scout image, at respective positions where upon execution of the actual scan, X-rays are applied in a body-axis direction of the subject and a view direction respectively and the X-rays transmitted through the subject are detected thereby to obtain X-ray projection data. Here, the central processing unit determines, based on the scout image, sectional areas of the subject and sectional shapes thereof so as to adapt to the respective positions for obtaining the X-ray projection data at the periphery of the subject upon execution of the actual scan. Thereafter, the central processing unit adjusts and sets respective set values of tube currents at the respective positions so as to adapt to the sectional areas and sectional shapes determined at the respective positions. Then, the set tube current values are supplied to the X-ray tube and the actual scan is executed on the subject to acquire X-ray projection data of the subject. Thereafter, a tomographic image of the subject is image-reconstructed based on the so-obtained projection data.
FIGS. 16(a), 16(b), 16(c) and 16(d) respectively show changes in X-ray tube current set upon execution of a helical scan. Here, the horizontal axis indicates a z-direction coordinate, and the vertical axis indicates an X-ray tube current value.
In FIG. 16(a), an X-ray tube current having a constant value is supplied to an X-ray tube to image or photograph respective portions or regions of a subject as viewed in a z-direction. In this case, there might be case in which the exposure of excessive X-rays occurs in the case of a region small in sectional area and in the case where a subject is child.
Therefore, as shown in FIG. 16(c), profile areas of the subject at respective z-direction positions are determined based on a scout image to take into consideration the sectional area of the subject in the z-direction. Thereafter, the set value of the X-ray tube current is optimized based on the determined profile areas such that image noise (standard deviation of CT value at each pixel) becomes approximately constant in the z-direction at each tomographic image.
Incidentally, in this case, the image noise of each tomographic image is set as a noise index value by inputting set values onto an input screen as shown in FIG. 14, for example.
The section of the subject is flat whose vertical and horizontal directions are not identical in length as in a round shape and whose aspect ratio differs. Therefore, when the center lines of the X-ray tube and multi-row X-ray detector are placed in the vicinity of the x-axis direction where a subject having a section of an elliptical shape long in an x-axis direction is photographed as shown in FIG. 16(b), X-ray tube current values are set larger than X-ray tube current values necessary in the case of a circular shape having the same area as the elliptical shape. On the other hand, when the center lines of the X-ray tube and multi-row X-ray detector are placed in the vicinity of the y-axis direction, X-ray tube current values are set smaller than X-ray tube current values necessary in the case of a circular shape having the same area as the elliptical shape. Thus, the X-ray tube current is changed continuously within view angles corresponding to 360°, whereby image noise contained in X-ray projection data in each view direction becomes approximately constant in each view direction of the subject.
That is, the X-ray tube current values are optimized in the z-direction as shown in FIG. 16(c), and the X-ray tube current values are optimized even within the xy plane as shown in FIG. 16(b), whereby the X-ray tube current values optimized based on three-dimensional information in the x, y and z-directions of the subject are set as shown in FIG. 16(d), thereby realizing an improvement in image quality.
However, for example, in the case when the subject is large, each tomographic image is thin in slice thickness, a scan speed is fast or a noise index value is small and good image quality is required, and so on, each X-ray tube current value set in the above-described manner becomes larger. Therefore, when an X-ray tube which is small in thermal capacity and needs to be cooled when a large X-ray tube current is outputted for long hours, or an X-ray tube which cannot output a large X-ray tube current, are mounted in the X-ray CT apparatus, there might be a case in which it is not possible to adapt to the set X-ray tube current values.
Thus, in such a case, there might be a case in which the above X-ray automatic exposure function cannot fulfill its full function.
In the X-ray CT apparatus including the multi-row X-ray detector or the two-dimensional X-ray area detector typified by the flat panel, the problem of X-ray needless exposure shows more increasing tendency. There has been a further demand for optimization of the image quality of each tomographic image due to the X-ray automatic exposure function.
Therefore, an object of the present invention is to provide an X-ray CT apparatus which adjusts other imaging or scanning condition parameters without depending on a restriction on a tube current value of an X-ray tube even though the tube current value is limited, thereby realizing image quality of a tomographic image corresponding to the optimum noise standard value at an X-ray automatic exposure function.
Another object of the present invention is to provide an X-ray CT apparatus which assigns priorities to plural parameters for imaging or scanning conditions influencing each photographed tomographic image and adjusts the plural parameters influencing the photographed tomographic image in order based on the priorities, thereby making it possible to realize the optimum image quality at an X-ray automatic exposure function.